Some important features in future cellular communications networks are higher bitrates and shorter delays applied to small cell scenarios. Higher bitrates can, for example, be achieved by using higher carrier frequencies where wideband spectrum resources are available. In addition, Time Division Duplexing (TDD), and in particular dynamic TDD, has attained an increased interest because downlink or uplink bitrates can be instantaneously increased by adaptively changing the relation between the number of intervals used for the downlink and the uplink.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 11 (Rel-11), the downlink is based on Orthogonal Frequency Division Multiplexing (OFDM) while the uplink is based on Discrete Fourier Transform (DFT) spread OFDM, i.e. Single Carrier Frequency Division Multiple Access (SC-FDMA), see, for example, 3GPP Technical Specification (TS) 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, V11.3.0. Here, the Transmission Time Interval (TTI) equals a subframe of 1 millisecond (ms), which consists of 14 OFDM symbols in the downlink and 14 SC-FDMA symbols in the uplink for the user data in the Physical Downlink Shared Channel (PDSCH) and the Physical Uplink Shared Channel (PUSCH) with normal length of the cyclic prefix. In future cellular communications networks, the length of a subframe might be significantly reduced in order to reduce user data delays. Also, the uplink and downlink ratios in future TDD systems may be significantly better optimized to various types of traffic supporting much larger uplink and downlink asymmetries than today. The switching between downlink and uplink is typically done on a subframe basis. Furthermore, in future cellular communications systems, both the downlink and the uplink might be based on the same radio access technology, such as, for example, OFDM or SC-FDMA.
When using TDD, the same frequency is used for both a downlink from a base station to a wireless device and an uplink from the wireless device to the base station. Both the wireless device and the base station must switch between transmitting and receiving, assuming that full duplex operation is not possible. Further, in 3GPP LTE Rel-11, a fixed allocation of uplink and downlink subframes is used, as specified in 3GPP TS 36.211 V11.3.0. A few predefined allocations are specified in 3GPP LTE Rel-11, as illustrated in FIG. 1. The number of uplink subframes for user data transmission in each 10 ms radio frame is between 1 and 6, which means a maximum of 60% of the total subframes in each radio frame can be used for uplink traffic. Here, a special subframe is inserted between downlink and uplink subframes, as illustrated in FIG. 2. The special subframe contains OFDM and SC-FDMA symbols for the downlink and the uplink, respectively, with a Guard Period (GP) in between. The guard period provides time for transmit and receive circuitry of the base station to switch from downlink transmission to uplink reception and time for transmit and receive circuitry of the wireless device to switch from downlink reception to uplink transmission.
The radio network node sends control signaling to the wireless device that includes a downlink assignment that indicates when and how the wireless device is scheduled to receive in the downlink and an uplink grant that indicates when and how the wireless device is to transmit in the uplink. In LTE, this control signaling is carried by either the Physical Downlink Control Channel (PDCCH) or the Enhanced PDCCH (EPDCCH). The downlink assignment is transmitted in the same subframe of the downlink in which the corresponding user data is transmitted. Conversely, the uplink grant is transmitted in the downlink a few subframes before the wireless device is scheduled to transmit in the uplink. More specifically, as illustrated in FIG. 3, if an uplink grant is transmitted in downlink subframe n, the wireless device can start uplink transmission in subframe n+g, where g is an uplink scheduling delay. For LTE TDD, the minimum uplink scheduling delay (g) is four subframes, which corresponds to 4 ms. However, the actual uplink scheduling delay depends on the uplink/downlink subframe allocation. So, in some cases, the uplink scheduling delay (g) can be larger than 4 ms. For instance, in the example of FIG. 3, an uplink grant is transmitted in the downlink in subframe 1 in order to grant radio resources for an uplink transmission in subframe 7. In this case, the uplink scheduling delay (g) is 6 subframes, which is equivalent to 6 ms. Likewise, in this example, an uplink grant is transmitted in the downlink in subframe 9 in order to grant radio resources for an uplink transmission in subframe 13. In this case, the uplink scheduling delay (g) is 4 subframes, which is equivalent to 4 ms. For more information regarding uplink scheduling, the interested reader is directed to 3GPP TS 36.213, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 11), V11.3.0.
Multiple Transmit Time Interval (multi-TTI) uplink grants are supported in LTE TDD uplink/downlink configuration 0. As illustrated in FIG. 4, a multi-TTI uplink grant schedules uplink transmissions in multiple uplink subframes. Currently, a maximum of two uplink subframes are scheduled with one multi-TTI uplink grant, as specified in 3GPP TS 36.213 V11.3.0. In the particular example of FIG. 4, a multi-TTI uplink grant is transmitted in the downlink in subframe 1 in order to grant uplink resources in subframes 7 and 8. Similarly, a multi-TTI uplink grant is transmitted in the downlink in subframe 5 in order to grant uplink resources in subframes 9 and 12.
In a dynamic TDD system, the relation between the number of downlink subframes and uplink subframes is not fixed, but can be flexibly configured depending of the current need. For example, the downlink/uplink configuration can be dynamically signaled, or alternatively, a wireless device can treat a subframe as a downlink subframe unless explicitly instructed to transmit in the uplink, as described in commonly held and assigned U.S. Pat. No. 8,559,343 entitled FLEXIBLE SUBFRAMES, issued Oct. 15, 2013.
In future cellular communications networks with high density deployment and higher carrier frequency, the coverage area of a radio network node can be small. Hence, the number of wireless devices connected to each radio network node in the network can be low and the traffic in the network can change dramatically. In the extreme case, the traffic for a given cell served by a radio network node can be only download traffic or only upload traffic at a given point in time. This presents new problems that are not addressed in the present 3GPP LTE standards.